Diurnal Freeze Research Articles

Effects of freeze-thaw cycles on the size distribution and stability of soil aggregate in the permafrost regions of the Qinghai-Tibetan Plateau

As the basic units of soil structure, soil aggregate is essential for maintaining soil stability. Intensified freeze–thaw cycles have deeply affected the size distribution and stability of aggregate under global warming. To date, it is still lacking about the effects of freeze–thaw cycles on aggregate in the permafrost regions of the Qinghai-Tibetan Plateau (QTP). Therefore, we investigated the effects of diurnal and seasonal freeze–thaw processes on soil aggregate. Our results showed that the durations of thawing and freezing periods in the 0–10 cm layer were longer than in the 10–20 cm layer, while the opposite results were observed during completely thawed and frozen periods. Freeze–thaw strength was greater in the 0–10 cm layer than that in the 10–20 cm layer. The diurnal freeze–thaw cycles have no significant effect on the size distribution and stability of aggregate. However, < 0.25 mm fraction dominated wet sieving aggregate with the highest proportion during thawing period, while the < 1 mm fraction reached the highest during completely frozen period in the 10–20 cm layer (P < 0.05). Likewise, the mean weight diameter and water-stable aggregate were decreased during thawing period compared with the other periods, which were influenced by soil microbial biomass carbon and belowground biomass. Hence, the seasonal freeze–thaw processes destroyed macro-aggregate (> 0.25 mm) and reduced aggregate stability. Our study has scientific guidance for evaluating the effects of freeze–thaw cycles on soil steucture and provides a theoretical basis for further exploration on soil and water conservation in the permafrost regions of the QTP.

Quantifying seasonal, depression focused recharge in the context of public supply well vulnerability

AbstractDepression focused recharge (DFR) may be a hydrologically important process that impacts the vulnerability of public supply wells, specifically related to pathogenic contaminants. The nature of DFR in glacial moraine environments, such as those located in northern latitudes within North America and Europe, is less well established than in other regions such as the Prairie Pothole Region (Northern United States, Western Canada) and the High Plains Aquifer (Central United States). The objectives of this study were to quantify seasonal infiltration flux beneath a topographically‐closed depression within 50 m of a public supply well and to interpret the impact of this DFR process on well vulnerability. Field instruments including groundwater monitoring wells, pressure transducers, soil moisture sensors and temperature sensors were installed in vertical clusters to capture the dynamics of infiltration, drainage and recharge within the depression feature. Continuous weather data were recorded by a meteorological station at the site. Transient infiltration was quantified during two contrasting hydrological events. The first event (~2 days) was an intense rainfall (&gt;50 mm) on a melting snowpack during the fall season when the soils were unfrozen. The second was a longer (35 day) period during the spring freshet when the surficial soils were initially frozen and subject to diurnal freezing and thawing and occasional precipitation events. The water table fluctuation method augmented by Darcy flux contributions, in addition to numerical modelling using the HYDRUS‐1D model, were used to quantify recharge rates beneath the depression. Numerical DFR estimates and analytical results differed by ±8%. Results indicate that recharge rates on the order of the annual regional average can occur beneath localized features in response to extreme events associated with snowmelt and intense rainfall. Such events may represent a microbial threat to groundwater quality if public supply wells are located nearby.

Soil microbial legacies influence freeze–thaw responses of soil

Abstract Warmer winters with less snowfall are increasing the frequency of soil freeze–thaw cycles across temperate regions. Soil microbial responses to freeze–thaw cycles vary and some of this variation may be explained by microbial conditioning to prior winter conditions, yet such linkages remain largely unexplored. We investigated how differences in temperature history influenced microbial community composition and activity in response to freeze–thaw cycles. We collected soil microbial communities that developed under colder (high elevation) and warmer (low elevation) temperature regimes in spruce‐fir forests, then added each of these soil microbial communities to a sterile bulk‐soil in a laboratory microcosm experiment. The inoculated high‐elevation cold and low‐elevation warm microcosms were subjected to diurnal freeze–thaw cycles or constant above‐freezing temperature for 9 days. Then, all microcosms were subjected to a 7‐day above‐freezing recovery period. Overall, we found that the high‐elevation cold community had, relative to the low‐elevation warm community, a smaller reduction in microbial respiration (CO2 flux) during freeze–thaw cycles. Further, the high‐elevation cold community, on average, experienced lower freeze–thaw‐induced bacterial mortality than the warm community and may have partly acclimated to freeze–thaw cycles via increased lipid membrane fluidity. Respiration of both microbial communities quickly recovered following the end of the freeze–thaw treatment period and there were no changes in soil extractable carbon or nitrogen. Our results provide evidence that past soil temperature conditions may influence the responses of soil microbial communities to freeze–thaw cycles. The microbial community that developed under a colder temperature regime was more tolerant of freeze–thaw cycles than the community that developed under a warmer temperature regime, although both communities displayed some level of resilience. Taken together, our data suggest that microbial communities conditioned to less extreme winter soil temperatures may be most vulnerable to rapid changes in freeze–thaw regimes as winters warm, but they also may be able to quickly recover if mortality is low. Read the free Plain Language Summary for this article on the Journal blog.

Effects of autumn diurnal freeze-thaw cycles on soil bacteria and greenhouse gases in the permafrost regions.

Understanding the impacts of diurnal freeze-thaw cycles (DFTCs) on soil microorganisms and greenhouse gas emissions is crucial for assessing soil carbon and nitrogen cycles in the alpine ecosystems. However, relevant studies in the permafrost regions in the Qinghai-Tibet Plateau (QTP) are still lacking. In this study, we used high-throughput pyrosequencing and static chamber-gas chromatogram to study the changes in topsoil bacteria and fluxes of greenhouse gases, including carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), during autumn DFTCs in the permafrost regions of the Shule River headwaters on the western part of Qilian Mountains, northeast margin of the QTP. The results showed that the bacterial communities contained a total of 35 phyla, 88 classes, 128 orders, 153 families, 176 genera, and 113 species. The dominant phyla were Proteobacteria, Acidobacteria, Actinobacteria, Chloroflexi, and Gemmatimonadetes. Two DFTCs led to a trend of increasing bacterial diversity and significant changes in the relative abundance of 17 known bacteria at the family, genus, and species levels. These were predominantly influenced by soil temperature, water content, and salinity. In addition, CO2 flux significantly increased while CH4 flux distinctly decreased, and N2O flux tended to increase after two DFTCs, with soil bacteria being the primary affecting variable. This study can provide a scientific insight into the impact of climate change on biogeochemical cycles of the QTP.

Spatial and temporal dynamics of aufeis in the Tso Moriri basin, eastern Ladakh, India

AbstractAufeis is a common phenomenon in cold regions of the Northern Hemisphere that develops during winter by successive water overflow and freezing on ice‐covered surfaces. Most studies on aufeis occurrence focus on regions in North America and Siberia, while research in High Mountain Asia (HMA) is still in an exploratory phase. This study investigates the extent and dynamics of icing processes and aufeis in the Tso Moriri basin, eastern Ladakh, India. Based on a combination of 235 Landsat 5 TM/8 OLI and Sentinel‐2 imagery from 2008 to 2021 the occurrence of icing and aufeis was classified using a random forest classifier. A total of 27 frequently occurring aufeis fields with an average maximum extent of 9 km2 were identified, located at a mean elevation of 4,700 m a.s.l. Temporal patterns show a distinct accumulation phase (icing) between November and April, and a melting phase lasting from May until July. Icing is characterized by high seasonal and inter‐annual variability. Successive water overflow mainly occurs between January and March and seems to be related to diurnal freeze–thaw‐cycles, whereas higher daytime temperatures result in larger icing areas. Aufeis feeding sources are often located within or in close vicinity to wetland areas, while vegetation is largely absent on surfaces with frequent aufeis formation. These interactions require more attention in future research. In addition, this study shows the high potential of a machine learning approach to monitor icing processes and aufeis, which can be transferred to other regions.

Year-long, broad-band, microwave backscatter observations of an alpine meadow over the Tibetan Plateau with a ground-based scatterometer

Abstract. A ground-based scatterometer was installed on an alpine meadow over the Tibetan Plateau to study the soil moisture and temperature dynamics of the top soil layer and air–soil interface during the period August 2017–August 2018. The deployed system measured the amplitude and phase of the ground surface radar return at hourly and half-hourly intervals over 1–10 GHz in the four linear polarization combinations (vv, hh, hv, vh). In this paper we describe the developed scatterometer system, gathered datasets, retrieval method for the backscattering coefficient (σ0), and results of σ0. The system was installed on a 5 m high tower and designed using only commercially available components: a vector network analyser (VNA), four coaxial cables, and two dual-polarization broad-band gain horn antennas at a fixed position and orientation. We provide a detailed description on how to retrieve the backscattering coefficients for all four linear polarization combinations σpq0, where p is the received and q the transmitted polarization (v or h), for this specific scatterometer design. To account for the particular effects caused by wide antenna radiation patterns (G) at lower frequencies, σ0 was calculated using the narrow-beam approximation combined with a mapping of the function G2/R4 over the ground surface. (R is the distance between antennas and the infinitesimal patches of ground surface.) This approach allowed for a proper derivation of footprint positions and areas, as well as incidence angle ranges. The frequency averaging technique was used to reduce the effects of fading on the σpq0 uncertainty. Absolute calibration of the scatterometer was achieved with measurements of a rectangular metal plate and rotated dihedral metal reflectors as reference targets. In the retrieved time series of σpq0 for L-band (1.5–1.75 GHz), S-band (2.5–3.0 GHz), C-band (4.5–5.0 GHz), and X-band (9.0–10.0 GHz), we observed characteristic changes or features that can be attributed to seasonal or diurnal changes in the soil: for example a fully frozen top soil, diurnal freeze–thaw changes in the top soil, emerging vegetation in spring, and drying of soil. Our preliminary analysis of the collected σpq0 time-series dataset demonstrates that it contains valuable information on water and energy exchange directly below the air–soil interface – information which is difficult to quantify, at that particular position, with in situ measurement techniques alone. Availability of backscattering data for multiple frequency bands (raw radar return and retrieved σpq0) allows for studying scattering effects at different depths within the soil and vegetation canopy during the spring and summer periods. Hence further investigation of this scatterometer dataset provides an opportunity to gain new insights in hydrometeorological processes, such as freezing and thawing, and how these can be monitored with multi-frequency scatterometer observations. The dataset is available via https://doi.org/10.17026/dans-zfb-qegy (Hofste et al., 2021). Software code for processing the data and retrieving σpq0 via the method presented in this paper can be found under https://doi.org/10.17026/dans-xyf-fmkk (Hofste, 2021).

Limited Response of Indigenous Microbes to Water and Nutrient Pulses in High-Elevation Atacama Soils: Implications for the Cold-Dry Limits of Life on Earth.

Soils on the world’s highest volcanoes in the Atacama region represent some of the harshest ecosystems yet discovered on Earth. Life in these environments must cope with high UV flux, extreme diurnal freeze–thaw cycles, low atmospheric pressure and extremely low nutrient and water availability. Only a limited spectrum of bacterial and fungal lineages seems to have overcome the harshness of this environment and may have evolved the ability to function in situ. However, these communities may lay dormant for most of the time and spring to life only when enough water and nutrients become available during occasional snowfalls and aeolian depositions. We applied water and nutrients to high-elevation soils (5100 meters above sea level) from Volcán Llullaillaco, both in lab microcosms and in the field, to investigate how microbial communities respond when resource limitations are alleviated. The dominant taxon in these soils, the extremophilic yeast Naganishia sp., increased in relative sequence abundance and colony-forming unit counts after water + nutrient additions in microcosms, and marginally in the field after only 6 days. Among bacteria, only a Noviherbaspirillum sp. (Oxalobacteraceae) significantly increased in relative abundance both in the lab and field in response to water addition but not in response to water and nutrients together, indicating that it might be an oligotroph uniquely suited to this extreme environment. The community structure of both bacteria and eukaryotes changed significantly with water and water + nutrient additions in the microcosms and taxonomic richness declined with amendments to water and nutrients. These results indicate that only a fraction of the detected community is able to become active when water and nutrients limitations are alleviated in lab microcosms, and that water alone can dramatically change community structure. Our study sheds light on which extremophilic organisms are likely to respond when favorable conditions occur in extreme earthly environments and perhaps in extraterrestrial environments as well.

Infiltration into frozen soil: From core‐scale dynamics to hillslope‐scale connectivity

AbstractInfiltration into frozen soil is a key hydrological process in cold regions. Although the mechanisms behind point‐scale infiltration into frozen soil are relatively well understood, questions remain about upscaling point‐scale results to estimate hillslope‐scale run‐off generation. Here, we tackle this question by combining laboratory, field, and modelling experiments. Six large (0.30‐m diameter by 0.35‐m deep) soil cores were extracted from an experimental hillslope on the Canadian Prairies. In the laboratory, we measured run‐off and infiltration rates of the cores for two antecedent moisture conditions under snowmelt rates and diurnal freeze–thaw conditions observed on the same hillslope. We combined the infiltration data with spatially variable data from the hillslope, to parameterise a surface run‐off redistribution model. We used the model to determine how spatial patterns of soil water content, snowpack water equivalent (SWE), and snowmelt rates affect the spatial variability of infiltration and hydrological connectivity over frozen soil. Our experiments showed that antecedent moisture conditions of the frozen soil affected infiltration rates by limiting the initial soil storage capacity and infiltration front penetration depth. However, shallow depths of infiltration and refreezing created saturated conditions at the surface for dry and wet antecedent conditions, resulting in similar final infiltration rates (0.3 mm hr−1). On the hillslope‐scale, the spatial variability of snowmelt rates controlled the development of hydrological connectivity during the 2014 spring melt, whereas SWE and antecedent soil moisture were unimportant. Geostatistical analysis showed that this was because SWE variability and antecedent moisture variability occurred at distances shorter than that of topographic variability, whereas melt variability occurred at distances longer than that of topographic variability. The importance of spatial controls will shift for differing locations and winter conditions. Overall, our results suggest that run‐off connectivity is determined by (a) a pre‐fill phase, during which a thin surface soil layer wets up, refreezes, and saturates, before infiltration excess run‐off is generated and (b) a subsequent fill‐and‐spill phase on the surface that drives hillslope‐scale run‐off.

Frost sorting on slopes by needle ice: A laboratory simulation on the effect of slope gradient

AbstractFollowing a previous attempt to reproduce miniature sorted patterns on a level surface, we report the results of a full‐scale laboratory simulation on frost sorting produced by needle ice activity on inclined surfaces. Four models, with different slope gradients (5°, 7°, 9°, 11°), were designed. Stones 6 mm in diameter placed in a grid covered 20% of the surface of frost‐susceptible water‐saturated soil. These models were subjected to 20–40 freeze–thaw cycles between 10°C and −5°C in 12 hours. The evolution of surface patterns was visually traced by photogrammetry. Needle ice growth and collapse induced downslope movement and concentrations of stones. A model produced incipient sorted circles on a 5° slope, whereas it resulted in three distinct sorted stripes on a 7° slope. The average diameter or spacing of these forms is 9.7–19.4 cm, comparable to those in the field dominated by diurnal freeze–thaw cycles. Surface parallel displacements of stone markers were traced with motion analysis software. The observed downslope stone displacements agree with those expected assuming that surface soil and stones move by repeated heaving perpendicular to the surface and vertical settlement due to gravity, although the growth of curved needle adds complexity to the overall displacements. Copyright © 2017 John Wiley &amp; Sons, Ltd.

Topographic controls on the debris-cover extent of glaciers in the Eastern Himalayas: Regional analysis using a novel high-resolution glacier inventory

To better understand the influence of topography on the formation of Himalayan debris-covered glaciers, we manually digitized 5301 glaciers, covering an area of 5691 ± 893 km2, from high-resolution Advanced Land Observing Satellite imagery, and identified 842 debris-covered glaciers with a debris-covered area of 3839 ± 753 km2. This estimation of the debris-covered area of these glaciers is a novel approach in the Eastern Himalayas. We investigated the relationship between the debris-covered area and the corresponding potential debris supply (PDS) slopes for the debris-covered glaciers across the region. The PDS slopes on the southern side show better correlation with their debris-covered area formation than those situated on the northern side of the Himalayan barrier, with this correlation weakening toward the western massifs. Further investigation showed that PDS slopes oriented SE to W exert a stronger control on the formation of debris-covered areas, which is related to the diurnal freeze–thaw cycle on the southern slope of the mountain barrier.

Seasonal changes in the sediment flux on steep hillslopes in a humid diurnal frost environment

AbstractMountain hillslopes are generally shaped by a combination of various types of sediment transport processes (e.g. surface erosion, soil creep, and dry ravel), which can occur simultaneously in the same area. Since sediment transport is generally affected by multiple microclimatic factors, such as heavy rainfall and changes in ground temperature, the types of predominant sediment transport processes vary by season. We conducted field observations in the southern Japanese Alps in the period from 2009 to 2013, using sediment traps and time lapse cameras, to investigate the seasonal changes in the type and flux of hillslope sediment transport on steep mountains in which both rainfall and diurnal freeze–thaw triggered sediment transport. In winter and early spring, sediment transport via diurnal freeze–thaw (soil creep and dry ravel) was highly active, whereas rainfall‐induced soil creep and selective transport by overland flow were active in summer and autumn when precipitation is abundant (average sediment flux of 0.033 and 0.074 kg m−1 day−1, respectively). Sediment flux was spatially variable and was affected by the form of the slope; sediment flux on the concave slope was higher than on the ridge‐shaped slope during both freeze–thaw and rainfall periods. Sediment flux on an old landslide exceeded that in second‐growth forest regardless of the slope shape. Temporal changes in the sediment flux were not completely synchronized among monitoring plots and were affected by slope shape, grain size, and episodic sediment supply events such as release of sediment from woody debris. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Impact of diurnal freeze–thaw cycles on the soil nematode Scottnema lindsayae in Taylor Valley, Antarctica

Global climate change scenarios predict not only higher temperatures, but also increased climatic variability. In cold regions, these changes may bring about a shift in the frequency of soil freeze–thaw cycles (FTCs), which represent a significant physiological challenge, especially for small, poikilothermic animals with limited mobility. To assess the impact of FTCs on cold-adapted soil biota, we evaluated freeze–thaw dynamics (i.e., 0 °C crossings) and demographics of the dominant nematode Scottnema lindsayae (proportion of adults, population size) over 20 years in soils at two locations in Taylor Valley, Antarctica. Based on hourly soil temperature data, we demonstrate that FTCs are a frequent feature in Taylor Valley, but with high inter-annual and spatial variability. Valley topography and soil moisture were found to impact FTC frequency, suggesting that basins within Taylor Valley have different susceptibilities to environmental variability. Increased FTC frequency in 1999–2001 coincided with a shift in S. lindsayae populations, with fewer juveniles reaching maturity. In the years following decreased adult proportions, overall S. lindsayae numbers were reduced, implying a strong negative effect of FTCs on in situ recruitment. Our results suggest that increased FTC frequency in the Dry Valleys slows S. lindsayae development, reducing reproductive success, and may take years to impact population size, which demonstrates the importance of long-term research to accurately predict the consequences of climate change on soil biota and biogeochemical cycling in the cold regions.

Laboratory frost sorting by needle ice: a pilot experiment on the effects of stone size and extent of surface stone cover

AbstractSorted patterned ground is ubiquitous where gravelly fine soil experiences freeze–thaw cycles, but experimental studies have rarely been successful in reproducing such patterns. This article reports an attempt to reproduce miniature sorted patterns by repeating needle‐ice formation, which simulates frost sorting in regions dominated by diurnal freeze–thaw cycles. Six full‐scale laboratory models were tested. They consisted of near‐saturated volcanic fine soil topped by small stones of uniform size; the models explored a range of stone size (~6, ~12, ~17 and ~22 mm) and surface abundance (20, 40 and 60% cover). The stones were placed in a grid on the surface. These models were subjected to 20–30 temperature excursions between 10 °C and −5 °C in 12 hours. The evolution of surface patterns were visually traced by photogrammetry. A data logging system continuously monitored vertical soil displacements, soil temperatures and moistures at different depths. All experimental runs displayed needle‐ice formation (2–3 cm in height) and resulting displacement of stones. The soil domains tended to heave faster and higher than the stones, leading to outward movement of the former and concentration of the stones. In plan view, smaller stones showed relatively fast and long‐lasting movements, while larger stones stabilized after the first five cycles. The 20% stone cover produced stone islands, whereas the 40% cover resulted in sorted labyrinths (a circle‐island complex) that may represent incipient sorted circles. The average diameter or spacing of these forms are 12–13 cm, being comparable to those in the field. The experiments imply that needle‐ice activity promotes rapid formation of sorted patterns, although the formation of well‐defined sorted circles may require hundreds of diurnal frost heave cycles. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Effects of freeze–thaw cycles on the soil nutrient balances, infiltration, and stability of cyanobacterial soil crusts in northern China

Freeze–thaw fluctuation is a natural phenomenon, which is frequently encountered by biological soil crusts (BSCs) in late autumn and early spring in cold deserts. The objective of our study was to investigate the effects of freeze–thaw cycles (FTCs) on the soil nutrient balances, infiltration, and stability of cyanobacterial soil crusts (CSCs) in the temperate desert region. A controlled incubation experiment was carried out to study the effects of diurnal freeze–thaw cycles (FTCs) on total soil carbon (TC), total soil nitrogen (TN), soil TC/TN, hydraulic conductivity, and strength of light and dark cyanobacterial crusts, respectively. Six successive diurnal FTCs were applied as three temperature regimes (i.e., six successive mild FTCs (mild), six successive severe FTCs (severe), three successive mild FTCs followed by three successive severe FTCs (medium)). The experiment intended to simulate natural temperature changes in one of the temperate regions of northern China. Compared with dark CSCs cores, light CSCs cores lost a greater proportion of nitrogen. For both crust cores, severe FTCs decreased TC and TN more than mild FTCs. However, TC and TN remained relative constant when CSCs cores were treated with severe FTCs after experiencing mild FTCs. TC and TN of both CSCs cores decreased in the earlier FTCs and then remained stable in the later FTCs. TC/TN increased significantly for light CSCs, but only changed slightly for dark CSCs after successive FTCs. The effects of FTCs on the hydraulic conductivity and strength of CSCs were not consistent with our expectations that FTCs would increase hydrological conductivity and decrease strength. These effects depended on crust type, FTC number, and freeze/thaw intensity. Increase in hydraulic conductivity and decrease in strength only occurred in severe treatment in the dark CSCs during the later FTCs. Light CSCs are more sensitive to FTCs than dark CSCs. Mild FTCs decrease less TC and TN than severe FTCs and mostly increase the stability of the CSCs. However, severe FTCs may decrease TC and TN drastically, thereby, degrading the BSCs.

The scale-dependence of SMOS soil moisture accuracy and its improvement through land data assimilation in the central Tibetan Plateau

This study evaluates SMOS (soil moisture and ocean salinity) soil moisture products against a newly established soil moisture network in the central Tibetan Plateau. Based on the results, the validity of assimilating the SMOS soil moisture retrievals into a land surface model is further evaluated. The ground truth is obtained by spatial upscaling from the network measurements within an area of approximately 10,000km2. Results show that both SMOS L2 and the preliminary version of L3 soil moisture products have large biases at the SMOS node scales (15 and 25km), but they can reflect the surface wetness conditions well when averaged at a 100-km scale during the unfrozen season (June to October). This finding indicates the applicability of SMOS retrievals is scale-dependent. Meanwhile, very few retrievals are available in winter due to the presence of frozen soil and snow cover, and the accuracy of ascending retrievals degrades during transition when diurnal freezing–thawing cycle occurs. Considering the SMOS L2 product has a better accuracy than that of L3, we assimilate it into a land surface model using a dual-pass land data assimilation scheme. The data assimilation estimate without in-situ tuning proves superior to either remote sensing or land surface modeling in estimating surface soil moisture for the unfrozen season, and its accuracy fulfills the SMOS measurement requirements (RMSD≤0.04m3m−3). Thus, the assimilation of SMOS retrievals holds promise to produce regional soil moisture dataset with acceptable accuracy for the Tibetan Plateau semi-arid region.

Southwest-facing slopes control the formation of debris-covered glaciers in the Bhutan Himalaya

Abstract. To understand the formation conditions of debris-covered glaciers, we examined the dimension and shape of debris-covered areas and potential debris-supply (PDS) slopes of 213 glaciers in the Bhutan Himalaya. This was undertaken using satellite images with 2.5 m spatial resolution for manual delineation of debris-covered areas and PDS slopes. The most significant correlation exists between surface area of southwest-facing PDS slopes and debris-covered area. This result suggests that the southwest-facing PDS slopes supply the largest quantity of debris mantle. The shape of debris-covered areas is also an important variable, quantitatively defined using a geometric index. Elongate or stripe-like debris-covered areas on north-flowing glaciers are common throughout the Bhutan Himalaya. In contrast, south-flowing glaciers have large ablation zones, entirely covered by debris. Our findings suggest that this difference is caused by effective diurnal freeze–thaw cycles rather than seasonal freeze–thaw cycles, permafrost degradation, or snow avalanches. In terms of geographic setting, local topography also contributes to glacier debris supply and the proportion of debris cover on the studied glaciers is suppressed by the arid Tibetan climate, whereas the north-to-south asymmetric topography of the Bhutan Himalaya has less influence on the proportion of debris cover.

Plant–plant interactions in tropical alpine environments

Plant–plant interactions are increasingly recognized as a key driver of community organization and ecosystem processes in alpine environments. However, patterns and mechanisms of plant–plant interactions remain largely uncharacterized in tropical alpine ecosystems (TAE) which represent as much as 10% of the total surface area of alpine ecosystems worldwide. In this paper, we review (1) the ecological and environmental features that are specific to TAE in comparison with other alpine ecosystems, (2) the existing literature on plant–plant interactions in TAE, and (3) whether patterns and mechanisms of plant–plant interactions established in extratropical alpine zones can be extended to TAE. TAE are located predominantly in South America, East Africa, and South-East Asia where they show a unique combination of environmental characteristics, such as absence of persisting snow cover, high frequency of diurnal freeze–thaw cycles and needle-ice activity, and a decrease in precipitation with increasing altitude. These environmental characteristics result in the presence of giant growth forms with a great architectural diversity. These biotic and abiotic characteristics influence the outcome of plant–plant interactions by imposing other types of environmental constraints than those found in extratropical alpine environments, and by potentially generating distinctive patterns of niche differentiation/complementarity between species and populations. To generalize the conceptual framework of plant–plant interactions in alpine environments, we advocate that TAE should be investigated more thoroughly by applying designs, methods and hypotheses that are used currently in temperate areas and by conducting studies along large latitudinal gradients that include tropical regions.

Effect of snowmelt time on growth and reproduction of Pedicularis davidii var. pentodon in the eastern Tibetan Plateau

The growth and reproduction of Pedicularis davidii Franch. var. pentodon Tsoong, a snowbank species in the eastern Tibetan Plateau, was studied along a snowmelt gradient of four segments. Soil water content in the snow-free period and amplitude of diurnal soil surface temperature in March to May were significantly different between the segments, whereas soil nutrients and pH showed no difference. The species did not occur at the early-melting segment. Stem height, leaf number per stem, leaf size and specific leaf area increased significantly along the remaining segments. Above-ground biomass and total biomass showed a similar trend, while below-ground biomass and the ratio of below- to above-ground biomass decreased along the gradient. Flower number, seeds per flower and 1000-seed weight increased in the lower and middle parts of an inflorescence, while they kept almost constant in upper part of an inflorescence along the gradient. As far as a whole inflorescence was concerned, these reproductive characteristics increased from early- to late-melting segments of the snowbank. Population density also increased along the snowmelt gradient. The distribution and growth pattern were mainly related to diurnal freeze–thaw cycles near the soil surface and soil water content.

Satellite microwave observations of the interannual variability of snowmelt on sea ice in the Southern Ocean

Snowmelt processes on Antarctic sea ice are examined. We present a simple snowmelt indicator based on diurnal brightness temperature variations from microwave satellite data. The method is validated through extensive field data from the western Weddell Sea and lends itself to the investigation of interannual and spatial variations of the typical snowmelt on Antarctic sea ice. We use in‐situ measurements of physical snow properties to show that despite the absence of strong melting, the summer period is distinct from all other seasons with enhanced diurnal variations of snow wetness. A microwave emission model reveals that repeated thawing and refreezing cause the typical microwave emissivity signatures that are found on perennial Antarctic sea ice during summer. The proposed melt indicator accounts for the characteristic phenomenological stages of snowmelt in the Southern Ocean and detects the onset of diurnal snow wetting. An algorithm is presented to map large‐scale snowmelt onset based on satellite data from the period between 1988 and 2006. The results indicate strong meridional gradients of snowmelt onset with the Weddell, Amundsen, and Ross Seas showing earliest (beginning of October) and most frequent snowmelt. Moreover, a distinct interannual variability of melt onset dates and large areas of first‐year ice where no diurnal freeze thawing occurs at the surface are determined.

Rockfall hazard in the Daisekkei Valley, the northern Japanese Alps, on 11 August 2005

This paper describes a rockfall event in the Daisekkei Valley of Mount Shirouma-dake (2,932 m), the northern Japanese Alps. The rockfall occurred on a steep cliff comprising well-jointed felsites and produced debris of ≥8,000 m3. Most debris was deposited on an elongated snowpatch located immediately beneath the cliff, and it caused casualties among people who were trekking along a trail on the snowpatch. Additionally, a large rock block slipped 1 km on the snowpatch. The rockfall could have been due to the differential retreat of the rockwall, which contains areas of high- and low-density joints. Seasonal and diurnal freeze–thaw activities and snow avalanches and wash appear to be important factors responsible for the retreat. Although some rock blocks that can collapse further remain on the rockwall, the position of the mountain trail in the Daisekkei Valley is fixed. Fundamental reform of tourism systems for climbers, including education on natural hazards, is required.